When people come to me and say they’re looking to buy a rainwater tank, the most important question I put to them is – ‘What do you want to do with the water you collect?’. This allows us to explore your expectations of what your tank will deliver for you and also ensures we can match the most appropriate tank(s) to suit your needs. While some of you already know what you want, we find a lot of people aren’t aware of certain things that can save them time and money in the long term. Here are 3 tips that will ensure you get what you want out of your rainwater harvesting system.
TIP 1: How Much Water Do You Need?
When I ask people what they intend to use their rainwater for, the three most common answers I get are to fill the toilet and/or washing machine, watering the garden and topping up the pool in the summertime. These are all great uses for rainwater to supplement the mains supply, but it doesn’t have to stop there. Our country cousins who don’t have access to the mains supply, will often use rainwater for their whole house supply and get to enjoy the taste of drinking water that hasn’t been treated with chemicals like the mains supply has. While we hear – ‘it’s recommended to use the mains supply if you have access to it’, we still have a choice to use our tank water in the best way we see fit. Recommendations and guidelines are just that, we can still make up our own minds.
Clearly defining what the rainwater will be used for allows you to select the most appropriate tank for your needs. If you just want to supplement the mains supply to your toilet and laundry, then a smaller 3,000 – 6,000 litre overall capacity can provide significant water savings when coupled to the house. If you want to be self sufficient, then you’ll likely be looking at storage over 100,000 litres and need the room to house one or more large tanks. On the other hand, you may just want to collect enough rainwater to make cups of tea or coffee and supply your own drinking water so you don’t have to buy it from the supermarket. Based on 8 cups of water a day (2 litres), each person in your household will consume 730 litres each year. Because we continually draw from our tanks, our tank capacity doesn’t have to match our annual consumption, unless we get all our rainfall in a very short, specific period of time.
When it comes to activities that require us to store water for dry periods such as watering the garden and topping up a swimming pool, careful consideration needs to be taken as replenishing the tank may take some time. A hand watered, mulched, water-wise garden will use a lot less water than an irrigated lawn and reticulated garden bed. Basic water pumps as well as the mains supply can often discharge around 30 litres per minute of water, so a 3,000 litre tank will be emptied in 100 minutes (1 hr and 40 minutes). If your pool gets topped up for 5 minutes a day, then the tank will be empty in 20 days time. Increasing your overall storage capacity will help deal with this, while using a pool cover will help reduce evaporation rates and make your tank water last longer. Wind and cooler night air has been found to be significant factors in pool evaporation rates, so covering the pool when it’s not being used is advised. It’s all about matching your tank storage capacity to your local conditions and needs.
TIP 2: How Much Water Can You Harvest?
The old saying of ‘save up for a rainy day’ or in this case, a dry period, has its benefits and limitations. By using rainwater on a regular basis when it’s raining, means we have room for more water to refill our tanks in the next rain event, increasing their efficiency. Managing our water tank usage relies on knowing when we receive our rainfall, then the amount we can harvest significantly increases. Looking up historical rainfall data in your area will show you the spread of rainfall over the year, with trends now showing a far greater spread over the months due to a changing climate. Keeping the tank full for a dry period means any more rain we get in-between will just flow straight out of the tank again and the potential to use it has been lost.
Finding out how much water you can harvest is a case of determining how much roof area can be diverted in to your tank(s) and how much rain you get in your location. This can be done with a tape measure to calculate the roof area per down-pipe (I’ve found each down-pipe generally feeds from approximately 35m² in and around Perth, Western Australia where we have an average annual rainfall of around 700mm) and a basic internet search of your local area for historical rainfall data (bom.gov.au in Australia). By multiplying the roof area (m²) to be used by the amount of millimetres (mm) of annual rainfall, the amount of water available to harvest can be calculated. We can then apply these findings to making a decision on how much rainwater storage we need.
You then need to take in to consideration dealing with the overflow when the tank is full. You don’t want to create a new problem for yourself by overloading a drainage system that can’t handle the outflow at one drainage point that was previously serviced by multiple drains. For tanks 5,000 litres and less, one or two downpipes to feed the tank is generally sufficient in areas that receive between 600 – 800mm of annual rainfall. Once again, this will also depend on how much water is being drawn out of the tank on a regular basis.
TIP 3: How Much Space Do You Have?
With the cost of land having risen so sharply in the last decade, it’s important that we make the most of what space we’ve got. Some people have a lot of space, so putting in a large round tank isn’t a problem for them. For the majority of us on suburban blocks, we still want to have room for a patio with a BBQ, the kids to run around on some lawn and a bit of a garden to add some colour or grow some fruit and vegetables in. Conventional round tanks have lost their appeal and slimline, under eaves tanks have enabled people to use dead spaces along the side of their homes in the fight for saving space.
There’s a variety of sizes and shapes of slimline tanks on the market today, all of which are just as effective at storing water as each other. When it comes to poly tanks (PE – polyethylene), they come in pre determined dimensions from the manufacturer due to costs associated with building the moulds to cast them. Corrugated iron tanks can be customised more in height and length, but getting the bend in the ends of the tank limits how narrow they can be made. They are still able to be made as narrow as many other tanks on the market and get lined with polymer to seal them. While concrete and fibreglass tanks are still common, they are generally round in shape, taking up valuable real estate in our back yards.
The next consideration is getting the tank in to position in the first place, which is where slimline tanks make life that little bit easier. I’ve only had to use a crane once to get a slimline tank in to place as they generally fit through gates and existing access ways. As a modern society, we’ve demanded choice and we have it when it comes to water tanks. When people ask me which tank is better, my reply is the one that leaves you the most space in your yard and the one you’re happy looking at for the next couple of decades. Your tank will most likely outlast your family car, so investing in a rainwater harvesting system that looks good and doesn’t impede on your yard or thoroughfare is by far the best decision in my mind.